In the case of belt sanders, be they longitudinal or wide belt sanders for the treatment of wood surfaces, the demand exists in many cases to improve only the quality of the surface by a uniform minor sanding, yet convex spots resulting from superficial unevenness should not be sanded off more vigorously than in relation to these spots of greater depth, as thereby the danger exists that thin veneer would be sanded all the way through at the convex spots. Thus, in these cases, what is required is no sanding to a uniform dimension in order to make the surface absolutely planar, but what is required is a uniform sanding off at all spots for an improvement of the surface quality which is equal everywhere.
This demand cannot be met with a rigid sliding contact over which the sanding belt moves. Hence, diverse measures are applied to achieve an elastic approach of the sanding belt against the surface to be treated. In the case of wide belt sanders, for instance, a sliding contact covered by a sliding coating is used, whereby an air cushion is generated between the sliding coating and the body of the sliding contact so that the sliding coating can be pressed in the form of an elastic cushion against the sanding belt treating the surface. However, no satisfactory results are obtained with this measure because thereby even the strongest pressure of the sanding belt takes place against the convex spots of the surface to be treated.
It already is known from Swiss Patent No. 477,263 and German Display Copy No. 20 23 540 to use a sliding contact provided on the running surface facing the rear side of the sanding belt with freely terminating air discharge nozzles so that a dynamic and not a stationary enclosed air cushion forms between sliding contact and sanding belt, which under certain conditions has the effect of actually adjusting the sanding belt to unevennesses of the blank surface as far as a range of about 1 millimeter, so that the intended uniform surface treatment can be accomplished. As results from German Display Copy No. 20 23 540, the sanding effect is a function of the thickness of the free air cushion as it is forming, which in turn can be influenced by the supply air pressure. Surprisingly, at higher supply air pressure which forms a thicker air cushion, substantially more uniform sanding off is accomplished, while at lower air pressure and with a thinner air cushion a calibrated sanding can be achieved for a uniform dimension of thickness.
One problem with this type of procedure resides in the machining of the edges of a blank contacted by the sanding belt and/or from where it moves away. German Display Copy No. 20 23 540 contains proposals for the solution of this problem. Even when mastering the problem of machining the edges, the method described in said document is practically unuseable for longitudinal belt sanders because the amount of seeping air at the sides of the sanding belt along the edges of the sliding contact is so high that energy-wise the operation of devices of this design is uneconomical.
The method described in German Display Copy No. 20 23 540 has been applied in connection with a wide belt sander. Because thereby the sanding belt operates via the longitudinal edges of the sliding contact, the consumption of air is substantially lower in the application with wide belt sanders. Depending on the thickness of the air cushion, and precisely with uniform surface sanding as mentioned above, a thicker air cushion is needed; however, considerable losses of air occur at the end of the sliding contact. For reasons of the critical processing of the edges of a blank, however, so far these losses of air were tolerated because the opinion prevailed that commensurate with the teachings of the German Display Copy No. 20 23 540, here an additional discharge of air is required to avoid producing an excessive edge sanding. Aside from the high consumption of air, which still is undesirable when using the wide belt sander, this method produces satisfactory results.